The invention of this application relates to non-metallic polymeric twist ties.
Throughout the specification, percentages of compositions are by weight and temperatures are in degrees Celsius unless indicated otherwise.
Twist ties comprising a middle wire centrally enclosed in a plastic or paper ribbon are ubiquitously used as closures, for instance to seal plastic bags, to fasten plants to stakes, to secure bundled electric cable, and for other fastening requirements. The widespread use of such ties results from the numerous advantageous properties. For instance, the same twist tie which can be applied mechanically, e.g. to bread bags and the like, in a high speed operation can also be applied manually in a somewhat slower speed operation with little physical exertion other than rotational twisting with the finger tips. Such metal twist ties can be multiply refastened with little reduction in fastening capability, for instance such ties can be reused up to ten times or more without failure. Moreover, such ties can be twisted without regard to directional rotation. In fact, such ties can be alternatively twisted in opposing rotational directions. Metal twist ties are also functional, i.e. can be tied, untied, retied and will hold with a secure twist, over a wide range of temperatures, e.g. from less than minus 10.degree. to greater than 65.degree..
Such metal twist ties are not universally used, however, for many food packaging applications because of certain disadvantageous properties. For instance, many convenience foods are packaged so that they can be heated in their original packaging in microwave ovens. Metal twist ties however will cause undesirable arcing when subjected to microwave radiation at an intensity common to such ovens.
In other cases it is common practice to inspect packaged food, e.g. sliced bread, for the possible presence of alien metal, e.g. chips, grit or filings from cutting blades or other mechanical equipment. In this regard it is desirable to inspect such sliced food products with metal detectors after final packaging. The use of metal twist ties hinder such practice. Accordingly, many sliced food products and microwavable convenience foods are packaged in plastic bags fastened by non-metallic closures, such as flat strip polymeric closures having bag neck confining apertures, such as disclosed in U.S. Pat. No. 3,164,250, or adhesive tapes. Flat strip polymeric closures are often undesirable because of their relatively high cost and inferior sealing capability. Adhesive tapes are undesirable because they are difficult to unfasten and generally have no refasten capability.
There have been a number of attempts to produce non-metallic polymeric twist ties with the desirable properties of metal twist ties. Such attempts have heretofore failed to replicate a sufficient number of the desirable properties of metal twist ties to provide a generally acceptable tie. For instance, polymeric ties have been prepared from plasticized polyvinylchloride ribbon containing up to about 20% or more of plasticizer. The effect of such high levels of plasticizer is to reduce the glass transition temperature of the polymer, e.g. to less than about 30.degree.. When such highly plasticized ties are exposed to temperatures near or above the glass transition temperature, twisted ties readily untwist. Such ties are effective only when tied into a knot.
Alternatively it has been proposed that polymeric twist ties be prepared from unplasticized polyvinylchloride. In this regard see Kirkpatrick who discloses in U.S. Pat. No. 3,565,738 polymeric ties in the form of a semi-rigid tape made of plastic material having a high tensile modulus and dead fold characteristics similar to those of a wire. Polymers disclosed by Kirkpatrick have been found to be unadaptable to mechanical twist tie apparatus.
Other non-metallic polymeric closures, e.g. for plastic bags, are disclosed in U.S. Pat. Nos. 3,334,805; 3,535,746; 3,604,066; 3,662,434; 3,945,086; 3,974,960 and 4,079,484.